Minor Components

Sterols: Sterols are nutritionally important lipids that need to be routinely determined in foods. 4-desmethylsterols, β-Sitosterol are the predominant sterols in olive oils. Minor sterols include ∆⁵-avenasterol, stigmasterol, sitostanol, and cholesterol.

The triterpene dialcohols erythrodiol and uvaol are also present in olive oil, at concentrations ranging from 10 to 200 mg/kg oil. The predominant sterol was β-sitosterol and total sterol content depended on the type of oil, and ranged from 687 to 2.479 mg/kg. Stigmasterol and the amount of erythrodiol plus uvaol can be used in order to distinguish between olive oil and seed oil (Martinez-Vidal, et al. 2007).

Compositional analysis of the sterol fraction of olive oil can be used to assess the degree of purity of the oil and the absence of other plant oils. This determination also permits characterization of the type of olive oil.

Squalene: Squalene is the major olive oil hydrocarbon and makes up more than 90% of the hydrocarbon fraction ranging from 200 to 7500 mg/kg oil or even higher (800−12000 mg/kg oil). Squalene is regarded as partially responsible for the beneficial effects of olive oil against certain cancers. In a recent study on the content of minor constituents of Italian olive oils, derived from olives of six cultivars and different degrees of ripeness, it was found that squalene loss during storage of oil samples in the dark was greater than that of α-tocopherol (Manzi, et al. 1998). This was attributed to a possible regeneration of α-tocopherol from squalene implying thus an antioxidant activity of this highly unsaturated hydrocarbon.

Pigments: The color of a virgin olive oil is due to the solubilization of the lipophilic chlorophyll and carotenoid pigments present in the source fruit. Virgin olive oil contains 1.0 to 2.7 ppm β-carotene as well as 0.9 to 2.3 ppm lutein (Psomiadou and Tsimidou 2002). Carotenoids and especially β-carotene can slow down oil oxidation by light filtering, singlet oxygen quenching, sensitizer inactivation, and free radical scavenging. Inthe absence of light, carotenoids and their oxidation products may act as prooxidants in vegetable oils (Velasco and Dobarganes 2002).

These components can transfer energy from light into chemical molecules. Thus, they act as prooxidants during storage in light. Although chlorophylls are strong prooxidants under light acting as a sensitizer to produce ¹O2, they act as antioxidants in

the dark possibly by donating hydrogen to free radical (Endo, et al. 1985, Francisca and Isabel 1992). Both chlorophylls and carotenoids are considered to have an important role in keeping the quality of edible oils, mainly due to their action as photo-sensitizers or singlet oxygen quenchers respectively (Cert, et al. 2000).

Tocopherols: There are four natural tocopherols α, β, γ, δ- forms are available in olive oil. The α-tocopherol or vitamin E is the major antioxidant present in olive oil and the amount is in the range of 150 and 300 ppm. These compounds display antioxidant properties and they are active as vitamins (vitamin E), which makes them particularly important for human health. The antioxidant activities are mainly depended on their concentration and presence of other antioxidants in olive oil. Tocopherols act as singlet oxygen quenchers and increase the oxidative stability of vegetable oils during storage in light and when chlorophyll is present (Cert, et al. 2000).

Phospholipids: The amount of phospholipids in olive oils changes between 40-135 mg/kg. Phosphatidylcholine, phospatidylethanolamine, phosphatidylinositol, and phosphatidylserine, phosphatidylglycerol, phosphatidic acid are the main phospholipids detected in olive oils. Their presence in the olive oils oils may affect their oxidative stability or the physicochemical state of cloudy (veiled) olive oil. The antioxidant functions of phospholipids based on an amino group that has the capacity to chelate metals and keep them in an active form. They can act as synergists with phenolic compounds and tocopherols contributing to enhance their antioxidant activity (Velasco and Dobarganes 2002). Phospholipids have hydrophilic and hydrophobic groups in the same molecule. The hydrophilic groups of the phospholipids are on the surface of oil and hydrophobic group are in the edible oil. The phospholipids decrease the surface tension of edible oil and may increase the diffusion rate of oxygen from the headspace to the oil to accelerate oil oxidation (Choe and Min 2006).

Phenolic Compounds: VOO contains at least 30 phenolic compounds. The major phenolic compounds are oleuropein derivatives, based on hydroxytyrosol which are strong antioxidants and radical scavengers. Phenolic compounds are complex class of chemicals including a hydroxyl group on a benzene ring. The pulp of olives contains these compounds, which are hydrophilic, but they are also found in the oil. The class of phenols includes numerous classes, such as simple phenolic acids and derivates like vanillic, coumaric and caffeic acids, tyrosol and hydroxytyrosol and more complex compounds like the secoiridoids of oleuropein and ligstroside, the lignans

(1-acetoxypinoresinol and pinoresinol), flavones (apigenin, luteolin). Table 2.2 refers to the major phenolic compounds in virgin olive oil.

Phenolic acids contains two distinguishing constitutive carbon frameworks, namely the hydroxycinnamic and hydroxybenzoic structures. They present in olives with the chemical structure of C6–C1 (benzoic acids) and C6–C3 (cinnamic acid) (Garrido Fernandez, et al. 1997). Phenolic acids have been associated with color and sensory qualities, as well as with the health-related and antioxidant properties of foods (Nergis and Unal 1991). Recent interest in phenolic acids stems from their potential protective role, through ingestion of fruit and vegetables, against diseases that may be related to oxidative damage (coronary heart disease, stroke, and cancers) (Masaki, et al.

1997). In particular, several phenolic acids such as gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, syringic, p- and o-coumaric, ferulic and cinnamic acid have been identified and quantified in VOO (in quantities lower than 1 mg of analyte kg^-1 of olive oil). Phenolic acids may be conjugated with organic acids, sugars, amino compounds, lipids, terpenoids, or other phenolics. Bianco, et al. (2002) investigated the presence of hydroxy-isochromans in VOO. In fact, during the malaxation step of VOO extraction, hydrolytic processes through the activity of glycosidases and esterases augment the quantity of hydroxytyrosol and carbonylic compounds, thus favouring the presence of all compounds necessary for the formation of isochroman derivatives. Two hydroxy-isochromans, formed by the reaction between hydroxytyrosol and benzaldehyde or vanillin, have been identified by HPLC-MS/MS technique and quantified in commercial VOOs.

The secoiridoids oleuropein, demethyloleuropein, and ligstroside are the main complex phenols in virgin olive oil. Secoiridoids are characterised by the presence of elenolic acid in its glucosidic or aglyconic form (Bianco and Uccella 2000). The secoiridoids, which are glycosidated compounds, are produced from the secondary metabolism of terpenes as precursors of several indole alkaloids (Soler-Rivas, et al.

2000) and are characterised by the presence of elenolic acid in its glucosidic or aglyconic form. Especially, they are formed from a phenyl ethyl alcohol (hydroxytyrosol and tyrosol), elenolic acid and, eventually, a glucosidic residue.

Oleuropein is the ester between 2-(3,4-dihydroxyphenyl) ethanol (hydroxytyrosol) and the oleosidic skeleton common to the glycosidic secoiridoids of the Oleaceae.

Oleuropein and demethyloleuropein are hydrolyzed by endogenous β-glycosidases to

the dialdehydic form of elenolic acid linked with 3,4-dihydroxyphenylethanol (3,4-DHPEA-EDA) and 3,4-dihydroxyphenylethanol-elenolic acid (3,4-DHPEA-EA) during crushing and malaxation (Bendini, et al. 2007).

Hydroxytyrosol, which is the major phenolic alcohol, can be present as a simple or esterified phenol with elenoic acid, forming oleuropein and its aglycone, or as part of the molecule of verbascoside (Servili, et al. 1999); it can also be present in several glycosidic forms, depending on the hydroxyl group to which the glucoside is bound (Bianco, et al. 1998, Ryan, et al. 2001).

Another group of substances present in the phenolic fraction is lignans, 1-acetoxypinoresinol and pinoresinol (Owen, et al. 2000). The substance (+)-pinoresinol is a common compound of the lignan fraction of several plants such as Forsythia species and Sesamum indicum seeds, while (+)-1 acetoxypinoresinol, (+)-1-hydroxypinoresinol and their glycosides have been found in the bark of the Olea europeae L. (olive) (Kato, et al. 1998).

The phenolic compounds in olive oil acted as antioxidants mainly at the initial stage of autoxidation (Deiana, et al. 2002) by scavenging free radicals and chelating metals. Changes in the phenolic compounds of virgin olive oils during storage are also reported. Cinquanta, et al. (1997) studied the evolution of simple phenols during 18 months of storage in the dark. They found a great increase in the tyrosol and hydroxytyrosol contents due to hydrolysis of their complex derivatives in a first stage and a rapid loss of hydroxytyrosol as compared with that of tyrosol at the end of the storage period. Hydroxytyrosol was the most effective antioxidant in olive oil oxidation.

Among phenolic compounds, o-diphenols such as caffeic acid are oxidized to quinones by ferric ions and become ineffective in inhibiting iron-dependent free radical chain reactions in oil (Keçeli and Gordon 2002). However, hydroxytyrosol, tyrosol, vanilic acid, p-coumaric acid were not oxidized by the ferric ions.

Flavonoids also have a great concern because of their beneficial health effects related to cancer and coronary heart diseases. Flavonoid aglycones are subdivided into flavones, flavonols, flavanones, and flavanols depending upon the presence of a carbonyl carbon at C-4, an OH group at C-3, a saturated single bond between C-2 and C-3, and a combination of no carbonyl at C-4 with an OH group at C-3, respectively.

Rovellini, et al. (1997) and Morello, et al. (2005) revealed the luteolin and apigenin in flavonoid group of phenolic compounds in VOO. Luteolin may originate from rutin or

Table 2.2. Major classes of phenolic compounds in VOO (Source: Servili, et al. 2004)

Major classes of phenolic compounds in VOO Phenolic acids and derivatives

Dialdehydic form of oleuropein aglycon Dialdehydic form of ligstroside aglycon Lignans

The concentrations of the phenolic compounds have a great importance through these compounds are responsible for the sensory and antioxidant properties of high-quality olive oils. The high-quality of the olives and the oil is affected by the amount of oleuropein and its hydrolytic products (Limiroli, et al. 1995). Separately, the absolute content of the phenolic compounds of the olive oil depends on the place of cultivation, the climate, the variety, and the olives’ level of maturation during of harvesting time (Cinquanta, et al. 1997, Visioli and Galli 1998, Brenes, et al. 1999).

Volatile Compounds: Volatile compounds are low molecular weight compounds which vapourise readily at room temperatures. Characteristic aroma and in particular green and fruity features of olive oil originates from many volatile compounds derived from the degradation of polyunsaturated fatty acids through a chain of enzymatic reactions known as the lipoxygenase (LOX) pathway which takes place during the oil extraction process (Angerosa, et al. 2000, Angerosa, et al. 2004).

Table 2.3. Defined major volatile compounds in VOO

aldehydes alcohols esters hydrocarbons ketones furans 3-Methylbutanal Methanol Methyl acetate Octane 2-Butanone Ethylfuran

(E)-2-Pentenal Ethanol Ethyl acetate 2-Methylbutane 3-Pentanone (Z)-2-Pentenal 1-Hexanol Butyl acetate Nonane 1-Penten-3-one

Hexanal 1-Penten-3-ol Hexyl acetate Hexane 2-Octanone (E)-2-Hexenal (E)-3-Hexen-1-ol (Z)-3-Hexenyl-acetate

(Z)-3-Hexenal (Z)-3-Hexen-1-ol Ethyl propanoate Heptanal (E)-2-Hexen-1-ol

2-4-Heptadienal 1-Octen-3-ol Octanal Terpineol Nonanal 3-Methylbutan-1-ol 2,4-Nonadienal

2,4-Decadienal

(E)-2-Undecenal

Some of the volatiles found in virgin olive oil are present in the intact tissue of the fruit, and others are formed during disruption of cell structure during the virgin olive oil production due to enzymatic reactions in the presence of oxygen. The main

precursors of volatile compounds are fatty acids (particularly linoleic and alpha-linolenic) and amino acids (leucine, isoleucine and valine) (Morales and Tsimidou 2000). In fact, it has been reported that the concentrations of volatile compounds depend on the enzymatic activity (Salas, et al. 2005) though external parameters (e.g. climate, soil, harvesting and extraction conditions) may alter the inherent olive oil sensory profile (Morales and Aparicio 1999).

The aroma of olive oil is attributed to aldehydes (hexanal, trans-2-hexenal, acetaldehyde), alcohols (methanol, hexan-1-ol, 3-methylbutan-1-ol), esters (methyl acetate, ethyl acetate, hexyl acetate), hydrocarbons (2-methylbutane, hexane, nonane), ketones (2-butanone, 3-methyl-2-butanone, 3-pentanone), furans and other undefined volatile compounds. The major volatiles in virgin olive oils are C6 and C5 volatile compounds (Angerosa, et al. 2004, Cimato, et al. 2006). Table 2.3 also demonstrates the major volatile compounds identified in VOO.

Volatile compounds, whether major or minor, contribute olive oil quality crucially and provide useful quality indicators. Beside volatile compounds, non-volatile compounds such as phenolic compounds also stimulate the tasting perception of bitterness, the latter pungency, astringency and metallic attributes.

In document Effect of storage time on olive oil quality (Page 27-33)